Just imagine. The XML serializer would need to know that the singular of children is child. Else you need some metadata to specify the extra information that is not modelled.

If you really want both children and child, you can very easily create an extra class accessed by a children property that has many elements accessed by a child property. But why bloat your model just to make XML, that should not normally be read by humans, cosmetically more pleasing?

(Ed Merks may give a better reply that explains how the ExtendedMetaData can be used to provide the extra metadata for your expectation.)

With extended metadata annotations on the Ecore model you can direct the serialization to produce "child" elements instead of "children" elements not but to introduce "fake" nesting level to group "child" elements under a "children element". Surely if you have an Xtext grammar the XML serialization is someone irrelevant?